Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, in the development of such batteries, designs of new electrodes and batteries to improve capacity density and specific energy are mainly studied.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte solution such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems such as ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated. Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries are solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity than those of the lithium ion batteries and an insufficient discharging capacity at low temperature, and therefore these disadvantages of the lithium-ion polymer batteries remain to be urgently solved.
Such electrochemical devices have been produced from many companies, and the battery stability has different phases in the electrochemical devices. Accordingly, it is important to ensure the stability of the electrochemical batteries. First of all, it should be considered that errors in operation of the electrochemical device should not cause damage to users. For this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical devices. In the stability characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin porous substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100° C. or above due to the features of its material and its manufacturing process such as elongation, so there may occur an electric short circuit between positive and negative electrodes.
In order to solve the above safety-related problems of the electrochemical device, Korean Laid-open Patent Publication No. 10-2006-72065 and No. 10-2007-231 disclose a separator 10 having a porous coating layer formed by coating at least one surface of a porous substrate 1 having many pores with a mixture of filler particles 3 such as inorganic particles and a binder polymer 5 (see FIG. 1). In the separator 10 having a porous coating layer, the filler particles 3 in the porous coating layer formed on the porous substrate 1 act as a kind of spacer that keeps a physical shape of the porous coating layer, so the filler particles 3 restrain thermal shrinkage of the porous substrate when the electrochemical device is overheated. In addition, interstitial volumes exist among the filler particles, thereby forming fine pores.
As mentioned above, the porous coating layer formed on the porous substrate contributes to the improvement of safety. In the related art, filler particles used for forming a porous coating layer have employed BaTiO3, Pb(Zr,Ti)O3 (PZT), ZrO2, SiO2, Al2O3, TiO2, lithium phosphate (Li3PO4) and lithium titanium phosphate (LixTiy(PO4)3, 0<x<2, 0<y<3), but these filler particles does not contribute to improvement of capacity of batteries due to their electrochemical characteristics. Thus, in case a porous coating layer is formed in a porous substrate with a common thickness, the separator is thickened, and an amount of electrode active material particles put into a unit volume is decreased accordingly, so a capacity per cell is decreased.
Meanwhile, in order to control thermal shrinkage of a porous substrate by the porous coating layer formed on the porous substrate, a sufficient amount of filler particles greater than a predetermined content should be contained therein. However, as the content of filler particles is increased, the content of polymer is relatively decreased. Accordingly, the filler particles of the porous coating layer may be disintercalated due to the stress occurring during the assembling process of an electrochemical device such as a taking-up process. The disintercalated filler particles act as a local defect of the electrochemical device, thereby giving a bad influence on the safety of the electrochemical device.